An increase in brain calcium has been demonstrated in dogs with acute uremia and was found to be induced by excess parathyroid hormone in blood. This abnormality is associated with deranged electroencephalogram (EEG). Prior removal of parathyroid glands prevents the accumulation of calcium in brain and normalized EEG. Studies will be carried out in dogs to evaluate the mechanisms by which calcium enters the brain and crosses the blood brain barrier in states with excess parathyroid hormone. More specifically, the following will be done: 1. Evaluate whether parathyroid hormone enters the brain as such, whether it stimulates an adenyl cyclase-cyclic AMP system in brain, and whether it enhances synthesis of a brain calcium binding protein; 2. Examine whether the calcium which entered the brain will leave the brain if the state of hyperparathyroidism is abolished; 3. Determine the relation between blood levels of parathyroid hormone and brain calcium. 4. Investigate whether high calcium content in brain is present in states with more chronic and prolonged hyperparathyroidism. 5. Examine whether calcitonin can antagonize the effect of parathyroid hormone on brain calcium.